2 ways of doing it really - you can change the port on the hosts, and forward those ports:

Server 1- 5666 (Standard)

Server 2 - 5667

Server 3 - 5668, etc.

Or you could setup NRPE to check those hosts from Server 1, and pass the result back. So on Server 1 setup a host check command, and then on Host 1 in Opsview check a create like, check_nrpe -H Server1 -c check_other_server -a Server2.

Its not very easy to manage; thats why Opsview has slave servers :) If you need this functionality, it sounds like you need a supported monitoring tool that come with the features you need - i.e. Pro or Enterprise.